Abstract:

Hydroformylation is an important commercial process for the conversion of alkenes, carbon monoxide and hydrogen into aldehydes to be further used in the production of various chemicals. The industrial processes operate in a homogeneous mode. Therefore, the development of a solid catalyst would solve problems related to catalyst separation. The purpose of this work was to study supported cobalt and rhodium catalysts in heterogeneous hydroformylation both in liquid and gas-phase conditions. The effect of different preparation methods, precursors, support modifications and pretreatments on the characteristics of the catalysts was investigated.

Atomic layer deposition (ALD) is a promising technique for the preparation of dispersed Co(A)/SiO2 catalysts using a Co(acac)3 precursor. Higher activity in ethene hydroformylation was obtained with Co(A)/SiO2 catalysts compared to impregnated Co(N)/SiO2 catalyst prepared from nitrate precursor. The dispersion, and consequently the activity and oxo-selectivity of the Co(A)/SiO2 catalyst, was further improved by inert handling of the catalyst. Moreover, by varying the metal content of the Co(A)/SiO2 catalysts, a clear correlation between metal dispersion and oxo-selectivity was found. The basic AlN modification of the silica support did not enhance hydroformylation activity due to low dispersion of the Co(A)/n·AlN/SiO2 catalysts.

For the carbon supported catalysts, the best hydroformylation activity was obtained with coconut-shell based Rh/C(C) catalyst. The presence of dispersed active sites and unreduced rhodium enhanced CO insertion, and also unintentional promotion by potassium was possible. Furthermore, without any pretreatment the catalyst exhibited even better propanal yields than with hydrogen pretreatment, apparently due to the better dispersed active sites. Pretreatment with carbon monoxide partially blocked the catalyst surface with carbonaceous residues, which improved CO insertion selectivity, but suppressed the overall activity.

The fibrous polymer-supported Rh-phosphine catalyst, Fibrecat™, prepared using a Rh(acac)(CO)2 precursor, was the most promising rhodium catalyst in ethene hydroformylation: high propanal selectivity (95%) and high activity were obtained under the mild reaction conditions of 100 °C and 0.5 MPa. The 31P NMR characterisations suggested the formation of both a Rh-monophosphine species, Rh(acac)(CO)(PS-PPh2), and a Rh-bisphosphine species, Rh(CO)2(PS-PPh2)2, on Fibrecat™, which were transformed in contact with CO/H2 to the active Rh-carbonyl hydrides.

In the liquid-phase hydroformylation of 1-hexene, the activity of the Rh/C catalysts appeared to correlate with the support: the larger the pores, the better the mass transfer and the higher the activity. In addition, C21 products were only formed on a support with sufficiently large pores – an indication of the heterogeneous functionality of the catalysts. With the carbonyl based cobalt catalysts, problems were encountered with the catalyst preparation and handling procedure due to the air sensitivity of the carbonyl precursors.